Mueller can't give them discovery--it gives the Russians too much info. So, he has to drop the charges. But the Russians can argue "You have besmirched our good name. We demand the right to defend ourselves."
Suppose the Judge hems and haws for a while and the Russians tell Mueller "Well, we won't go any further if you give us a statement that the indictment was a mistake and you now believe that there was no collusion."
Now Mueller is really stuck. If he says "no collusion" his entire reason for being disappears. If he says "collusion" they get discovery.
Good thinking and analysis.